1. Field of the Invention
The present invention relates to a radiation imaging system that carries out radiography with the use of radiation such as X-rays. More specifically, the present invention relates to the radiation imaging system for phase imaging that is provided with gratings disposed between a radiation source and a radiation image detector, and a method for detecting positional deviation between the gratings.
2. Description Related to the Prior Art
X-rays are used as a probe for imaging inside of an object without incision, due to the characteristic that attenuation of the X-rays depends on the atomic number of an element constituting the object and the density and thickness of the object. Radiography with the use of the X-rays is widely available in fields of medical diagnosis, nondestructive inspection, and the like.
In a conventional X-ray imaging system for capturing an X-ray image of the object, the object to be examined is disposed between an X-ray source for emitting the X-rays and an X-ray image detector for detecting the X-rays. The X-rays emitted from the X-ray source are attenuated (absorbed) in accordance with the characteristics (atomic number, density, and thickness) of material of the object present in an X-ray path, and are then incident upon pixels of the X-ray image detector. Thus, the X-ray image detector detects the X-rays, and the X-ray image is formed from a detection signal. There are some types of X-ray image detectors in widespread use, such as a combination of an X-ray intensifying screen and a film, an imaging plate containing photostimulated phosphor, and a flat panel detector (FPD) that is composed of semiconductor circuits.
The smaller the atomic number of the element constituted of the material, the lower X-ray absorptivity the material has. Thus, the X-ray image of living soft tissue, soft material, or the like, cannot have sufficient contrast. Taking a case of an arthrosis of a human body as an example, both of cartilage and joint fluid surrounding the cartilage have water as a predominant ingredient, and little difference in the X-ray absorptivity therebetween. Thus, the X-ray image of the arthrosis hardly has sufficient contrast.
With this problem as a backdrop, X-ray phase imaging is actively researched in recent years. In the X-ray phase imaging, an image (hereinafter called phase contrast image) is obtained based on phase shifts (shifts in angle) of the X-rays that have passed through the object, instead of intensity distribution of the X-rays having passed therethrough. It is generally known that when the X-rays are incident upon the object, the phases of the X-rays interact with the material more closely than the intensity of the X-rays. Accordingly, the X-ray phase imaging, which takes advantage of phase difference, allows obtainment of the image with high contrast, even in capturing the image of the object constituted of the materials that have little difference in the X-ray absorptivity. As a type of the X-ray phase imaging, an X-ray imaging system using an X-ray Talbot interferometer, which is constituted of the X-ray source, two transmission diffraction gratings, and the X-ray image detector, is devised in recent years (refer to Japanese Patent Laid-Open Publication No. 2008-200360 and Applied Physics Letters, Vol. 81, No. 17, page 3287, written on October 2002 by C. David et al., for example).
The X-ray Talbot interferometer is constituted of the X-ray source, the X-ray image detector, and first and second diffraction gratings disposed between the X-ray source and the X-ray image detector. The second diffraction grating is disposed downstream from the first diffraction grating by a Talbot distance, which is determined from a grating pitch of the first diffraction grating and the wavelength of the X-rays. The Talbot distance is a distance at which the X-rays that have passed through the first diffraction grating form a self image by the Talbot effect. This self image is distorted and deformed according to the phase shifts of the X-rays due to passage through the object. By overlaying the second diffraction grating on the deformed self image of the first diffraction grating, i.e. by intensity modulation of the self image, moiré fringes appear.
The X-ray Talbot interferometer detects the moiré fringes by a fringe scanning technique. The phase contrast image of the object is obtained from change of the moiré fringes by the object. In the fringe scanning technique, a plurality of images are captured, while the second diffraction grating is slid relative to the first diffraction grating in a direction substantially parallel to a surface and orthogonal to a grating direction of the first diffraction grating at a scan pitch, which corresponds to an equally divided part of a grating pitch. By this scanning operation, the X-ray image detector detects periodic change in the intensity of pixel data of each pixel. From a phase shift amount (a phase shift amount between the presence and the absence of the object) of the periodic change in the intensity, a differential phase image (corresponding to angular distribution of the X-rays refracted by the object) is obtained. Integration of the differential phase image along a fringe scanning direction allows obtainment of the phase contrast image. Since the pixel data is a signal the intensity of which is periodically modulated in the scanning operation, a set of the pixel data obtained by the scanning operation is hereinafter called “intensity modulation signal”. This fringe scanning technique is also adopted in an imaging system using laser light instead of the X-rays (refer to Applied Optics, Vol. 37, No. 26, page 6227, written on September 1998 by Hector Canabal et al.).
The X-ray imaging system using the X-ray Talbot interferometer requires high alignment accuracy between the first and second diffraction gratings, and tolerance for positional deviation in the grating direction (fringe scanning direction) is extremely small. Japanese Patent Laid-Open Publication No. 2008-200360 proposes to detect a positional deviation amount between the first and second diffraction gratings by an accelerometer and a position sensor, and to make a notification if the positional deviation amount is out of predetermined bounds.
However, the Japanese Patent Laid-Open Publication No. 2008-200360 needs provision of the accelerometer and the position sensor for detecting the positional deviation amount between the first and second diffraction gratings, and moreover needs provision of controllers for the accelerometer and the position sensor. Thus, the X-ray imaging system becomes complicated and expensive.
The Japanese Patent Laid-Open Publication No. 2008-200360 describes that above imaging operation is carried out without disposing the object between the X-ray source and the X-ray image detector, to calculate an offset amount Δ(x, y) of the differential phase image caused by a positional error between the first and second diffraction gratings and the like. However, the patent document does not describe to detect the positional deviation amount between the first and second diffraction gratings from the offset amount Δ(x, y). Also, when the positional deviation amount is large, the contrast of the intensity modulation signal is reduced. Thus, it becomes difficult to precisely calculate the positional deviation amount from the offset amount Δ(x, y).
Furthermore, the Japanese Patent Laid-Open Publication No. 2008-200360 aims to detect the positional deviation between the first and second diffraction gratings that occurs during application of the X-rays. The document does not describe the detection of the positional deviation due to time degradation.